Refactor getobj() to use callbacks on candidate objects

This replaces the arcane system previously used by getobj where the
caller would pass in a "string" whose characters were object class
numbers, with the first up to four characters being special constants
that effectively acted as flags and had to be in a certain order.
Because there are many places where getobj must behave more granularly
than just object class filtering, this was supplemented by over a
hundred lines enumerating all these special cases and "ugly checks", as
well as other ugly code spread around in getobj callers that formatted
the "string".

Now, getobj callers pass in a callback which will return one of five
possible values for any given object in the player's inventory. The
logic of determining the eligibility of a given object is handled in the
caller, which greatly simplifies the code and makes it clearer to read.
Particularly since there's no real need to cram everything into one if
statement.

This is related to pull request #77 by FIQ; it's largely a
reimplementation of its callbacks system, without doing a bigger than
necessary refactor of getobj or adding the ability to select a
floor/trap/dungeon feature with getobj. Differences in implementation
are mostly minor:
- using enum constants for returns instead of magic numbers
- 5 possible return values for callbacks instead of 3, due to trying to
  make it behave exactly as it did previously. PR #77 would sometimes
  outright exclude objects because it lacked semantics for invalid
  objects that should be selectable anyway, or give slightly different
  messages.
- passing a bitmask of flags to getobj rather than booleans (easier to
  add more flags later - such as FIQ's "allow floor features" flag, if
  that becomes desirable)
- renaming some of getobj's variables to clearer versions
- naming all callbacks consistently with "_ok"
- generally more comments explaining things

The callbacks use the same logic from getobj_obj_exclude,
getobj_obj_exclude_too and getobj_obj_acceptable_unlisted (and in a few
cases, from special cases still within getobj). In a number of them, I
added comments suggesting possible further refinements to what is and
isn't eligible (e.g. should a bullwhip really be presented as a
candidate for readying a thrown weapon?)

This also removed ALLOW_COUNT and ALLOW_NONE, relics of the old system,
and moved ALLOW_ALL's definition into detect.c which is the only place
it's used now (unrelated to getobj). The ALLOW_ALL functionality still
exists as the GETOBJ_PROMPT flag, because its main use is to force
getobj to prompt for input even if nothing is valid.

I did not refactor ggetobj() as part of this change.

src/eat.c

@@ -30,6 +30,7 @@ static void FDECL(fprefx, (struct obj *));
 static void FDECL(fpostfx, (struct obj *));
 static int NDECL(bite);
 static int FDECL(edibility_prompts, (struct obj *));
+static int FDECL(tinopen_ok, (struct obj *));
 static int FDECL(rottenfood, (struct obj *));
 static void NDECL(eatspecial);
 static int FDECL(bounded_increase, (int, int, int));
@@ -38,6 +39,9 @@ static void FDECL(eataccessory, (struct obj *));
 static const char *FDECL(foodword, (struct obj *));
 static int FDECL(tin_variety, (struct obj *, BOOLEAN_P));
 static boolean FDECL(maybe_cannibal, (int, BOOLEAN_P));
+static int FDECL(eat_ok, (struct obj *));
+static int FDECL(offer_ok, (struct obj *));
+static int FDECL(tin_ok, (struct obj *));
 
 /* also used to see if you're allowed to eat cats and dogs */
 #define CANNIBAL_ALLOWED() (Role_if(PM_CAVE_DWELLER) || Race_if(PM_ORC))
@@ -55,18 +59,6 @@ static boolean FDECL(maybe_cannibal, (int, BOOLEAN_P));
 #define nonrotting_food(otyp) \
     ((otyp) == LEMBAS_WAFER || (otyp) == CRAM_RATION)
 
-static NEARDATA const char comestibles[] = { FOOD_CLASS, 0 };
-static NEARDATA const char offerfodder[] = { FOOD_CLASS, AMULET_CLASS,
-                                                 0 };
-
-/* Gold must come first for getobj(). */
-static NEARDATA const char allobj[] = {
-    COIN_CLASS,   WEAPON_CLASS, ARMOR_CLASS,  POTION_CLASS,
-    SCROLL_CLASS, WAND_CLASS,   RING_CLASS,   AMULET_CLASS,
-    FOOD_CLASS,   TOOL_CLASS,   GEM_CLASS,    ROCK_CLASS,
-    BALL_CLASS,   CHAIN_CLASS,  SPBOOK_CLASS, 0
-};
-
 /* see hunger states in hack.h - texts used on bottom line */
 const char *hu_stat[] = { "Satiated", "        ", "Hungry  ", "Weak    ",
                           "Fainting", "Fainted ", "Starved " };
@@ -102,7 +94,6 @@ register struct obj *obj;
         && !Has_contents(obj))
         return TRUE;
 
-    /* return (boolean) !!index(comestibles, obj->oclass); */
     return (boolean) (obj->oclass == FOOD_CLASS);
 }
 
@@ -2763,6 +2754,18 @@ doeat()
     return 1;
 }
 
+/* getobj callback for object to be opened with a tin opener */
+static int
+tinopen_ok(obj)
+struct obj *obj;
+{
+    if (obj && obj->otyp == TIN)
+        return GETOBJ_SUGGEST;
+
+    return GETOBJ_EXCLUDE;
+}
+
+
 int
 use_tin_opener(obj)
 struct obj *obj;
@@ -2788,7 +2791,7 @@ struct obj *obj;
         res = 1;
     }
 
-    otmp = getobj(comestibles, "open");
+    otmp = getobj("open", tinopen_ok, GETOBJ_NOFLAGS);
     if (!otmp)
         return res;
 
@@ -3163,6 +3166,60 @@ boolean incr;
     }
 }
 
+/* getobj callback for object to eat - effectively just wraps is_edible() */
+static int
+eat_ok(obj)
+struct obj *obj;
+{
+    if (!obj)
+        return GETOBJ_EXCLUDE;
+
+    if (is_edible(obj))
+        return GETOBJ_SUGGEST;
+
+    /* make sure to exclude, not downplay, gold (if not is_edible) in order to
+     * produce the "You cannot eat gold" message in getobj */
+    if (obj->oclass == COIN_CLASS)
+        return GETOBJ_EXCLUDE;
+
+    return GETOBJ_EXCLUDE_SELECTABLE;
+}
+
+/* getobj callback for object to be offered (corpses and things that look like
+ * the Amulet only */
+static int
+offer_ok(obj)
+struct obj *obj;
+{
+    if (!obj || (obj->oclass != FOOD_CLASS && obj->oclass != AMULET_CLASS))
+        return GETOBJ_EXCLUDE;
+
+    if (obj->otyp != CORPSE && obj->otyp != AMULET_OF_YENDOR
+        && obj->otyp != FAKE_AMULET_OF_YENDOR)
+        return GETOBJ_EXCLUDE_SELECTABLE;
+
+    /* suppress corpses on astral, amulets elsewhere
+     * (!astral && amulet) || (astral && !amulet) */
+    if (Is_astralevel(&u.uz) ^ (obj->oclass == AMULET_CLASS))
+        return GETOBJ_DOWNPLAY;
+
+    return GETOBJ_SUGGEST;
+}
+
+/* getobj callback for object to be tinned */
+static int
+tin_ok(obj)
+struct obj *obj;
+{
+    if (!obj || obj->oclass != FOOD_CLASS)
+        return GETOBJ_EXCLUDE;
+
+    if (obj->otyp != CORPSE || !tinnable(obj))
+        return GETOBJ_EXCLUDE_SELECTABLE;
+
+    return GETOBJ_SUGGEST;
+}
+
 /* Returns an object representing food.
  * Object may be either on floor or in inventory.
  */
@@ -3278,11 +3335,18 @@ int corpsecheck; /* 0, no check, 1, corpses, 2, tinnable corpses */
     }
 
  skipfloor:
-    /* We cannot use ALL_CLASSES since that causes getobj() to skip its
-     * "ugly checks" and we need to check for inedible items.
-     */
-    otmp = getobj(feeding ? allobj : offering ? offerfodder : comestibles,
-                  verb);
+    /* We cannot use GETOBJ_PROMPT since we don't want a prompt in the case
+     * where nothing edible is being carried. */
+    if (feeding)
+        otmp = getobj("eat", eat_ok, GETOBJ_NOFLAGS);
+    else if (offering)
+        otmp = getobj("sacrifice", offer_ok, GETOBJ_NOFLAGS);
+    else if (corpsecheck == 2)
+        otmp = getobj(verb, tin_ok, GETOBJ_NOFLAGS);
+    else {
+        impossible("floorfood: unknown request (%s)", verb);
+        return (struct obj *) 0;
+    }
     if (corpsecheck && otmp && !(offering && otmp->oclass == AMULET_CLASS))
         if (otmp->otyp != CORPSE || (corpsecheck == 2 && !tinnable(otmp))) {
             You_cant("%s that!", verb);